基于小试样的定向凝固镍基合金蠕变性能评估方法

在热处理态定向凝固镍基合金棒材上制取工字型非标准小试样和棒状蠕变标准试样,并进行了不同温度(800,850,900,980℃)下的应力松弛试验和不同应力(标准试样为250,350,450 MPa,小试样为250,350,400,450 MPa)下的蠕变持久试验,研究了标准试样和小试样的松弛蠕变和持久蠕变行为,建立了小试样松弛蠕变速率与标准试样持久寿命的关系。结果表明：小试样和标准试样的持久蠕变曲线均单调递增直至试样断裂,持久蠕变速率曲线均均呈“U”字型,两者的最小持久蠕变速率接近;小试样和标准试样的高温松弛蠕变曲线均单调递减,温度归一化后两者松弛蠕变速率分布在一条线上;持久蠕变和应力松弛试验后标准试样和小试样中γ′相均沿垂直于应力方向聚集粗化,属于N型筏化;标准试样持久寿命与小试样松弛蠕变速率存在线性关系,利用小试样松弛蠕变速率预测的持久寿命和标准试样实际持久寿命接近。     

钛合金材料IMI834高温蠕变和蠕变断裂的连续损伤力学分析

对工作在650℃下的航空发动机用高温钛合金材料IMI834的高温蠕变和蠕变断裂行为进行连续损伤力学分析研究,预测结构件的蠕变断裂寿命,得到的结果和试验数据相吻合。用有限元软件ANSYS分析半圆形缺口圆棒试样的应力应变状态,得到应力三轴因子,进而考虑多轴应力状态对材料蠕变断裂寿命的影响,表明多轴应力显著地降低结构的蠕变断裂寿命。分析结果显示,文中所用的蠕变损伤模型能够合理地描述材料蠕变损伤的累积发展过程,并正确地预测结构件的蠕变断裂寿命。     

“服役条件—持久强度”(SCRI)干涉模型及其在持久寿命可靠性预测中的应用

对于高温构件持久寿命的可靠性预测而言,材料持久性能的分散以及服役条件如温度和应力的波动对可靠性评估结果有直接的影响。文中提出基于Z参数的"服役条件—持久强度"干涉模型("service condition-creep rupture prop-erty"interference model,SCRI模型)用于高温材料持久寿命的可靠性预测。利用Z参数方法,高温材料持久强度的分散性服从正态分布,而服役温度和应力波动造成的服役条件的分布可以用MonteCarlo方法模拟获得,从而实现在考虑性能数据分散性以及服役条件波动性的情况下材料持久寿命的可靠性分析。文中提出的干涉模型为高温蠕变持久寿命的可靠性评估提供新的思路和方法。     

30Cr1Mo1V转子钢应变控制下的蠕变--疲劳交互作用研究

在540 ℃和565 ℃下进行了30Cr1Mo1V带有保持时间的总应变控制轴向高温低周疲劳试验,保持时间 10～60 s,应变幅为0.6%～1.2%,采用拉应对称梯形波.对该材料蠕变-疲劳交互作用下的应力松弛现象及应变-寿命规律进行了研究.选取了几个常用的蠕变-疲劳寿命预测模型对试验数据进行分析,对各种预测模型的准确性进行了寿命预测因子和标准偏差分析.试验结果表明,随着控制应变幅的增加,第一、三阶段比例均呈增加趋势,第二阶段的比例呈现减小趋势;以应力下降比率作为参考变量,在整个寿命周期内,拉应力松弛与压应力松弛基本一致,应力下降比率基本相同.采用线性累计损伤法和应变范围划分法的准确度较高,而频率修正法和应变能划分法的准确度较差;给出了各种方法的寿命预测公式.     

高温构件蠕变损伤与裂纹扩展预测研究新进展

精准的寿命预测是高温构件设计制造与运行维护的关键,但多轴应力和裂纹等缺陷的存在使得寿命预测的难度大大增加。综述了笔者近年来在高温蠕变损伤模型和蠕变裂纹扩展仿真方面的研究工作,主要包括:讨论了应力水平和应力状态对蠕变断裂应变的影响规律;基于幂律蠕变控制孔洞长大理论,提出了新的多轴蠕变延性模型;采用基于应变的损伤力学模型,预测了多种含缺陷结构中蠕变裂纹的扩展行为,并分析了蠕变条件下多个表面裂纹干涉、扩展及合并的全过程;发展了基于晶界孔洞化损伤机制的裂纹扩展分析方法,实现了蠕变疲劳裂纹扩展仿真和蠕变疲劳氧化裂纹扩展仿真。这些工作为建立考虑多轴应力影响的含缺陷高温构件寿命预测方法提供了有力支持。     

一种2.25Cr1MoV钢加氢反应设备蠕变疲劳寿命设计方法

加氢反应器在服役过程中会受到高温高压和循环载荷的共同作用,设计时必须同时考虑蠕变和疲劳的影响。ASME规范案例2605提供一种针对2.25Cr1MoV钢的蠕变疲劳寿命设计方法。借助有限元分析软件ANSYS,采用案例2605中的方法对加氢反应设备圆柱筒体直接管结构进行蠕变疲劳寿命校核,详细阐述该案例的设计思想和具体步骤,并讨论操作温度和压力对于设计寿命的影响。结果表明,接管结构蠕变疲劳寿命校核满足要求,筒体与直接管连接处出现应力集中,由于蠕变应变的产生,应力集中处的应力值随蠕变时间的增加不断下降,蠕变损伤累积速度也随之下降。同时,提高操作温度和操作压力均会造成蠕变损伤的增加,疲劳应力幅对蠕变疲劳的寿命影响较大。针对模拟结果,提出一些提高结构蠕变疲劳寿命的措施。     

蠕变持久寿命幂函数预测方法

通过对大量试验数据的分析研究发现．蠕变持久寿命试验数据经加权累加变换后,能够很好地呈现出幂函数变化规律。由此建立蠕变持久寿命幂函数预测模型,并采用导数最小二乘法确定模型中的待定参数。该方法不但拟合蠕变持久曲线本身,而且还同时拟合蠕变持久曲线上各点的导数,能够很好地描述蠕变持久曲线及其变化趋势,从而实现长期蠕变持久寿命的高精度预测。大量对比试验结果表明,与传统方法相比,蠕变持久寿命幂函数预测方法可以获得更高的预测精度。文中给出两个应用实例。     

基于蠕变公式的时效应力松弛行为预测模型

研究时效成形过程蠕变与应力松弛行为的关联规律及其转换模型。通过分析金属材料蠕变与应力松弛行为的产生机理及其相互关系,分别建立基于加工硬化假说和时效假说的蠕变-应力松弛基本转换模型。在此基础上,针对蠕变/应力松弛时效成形过程将蠕变与时效同步进行的特征,进一步建立基于蠕变公式的时效应力松弛行为预测模型。以铝合金7055T6为试验材料,在120℃时效温度下分别进行不同应力水平下的蠕变与应力松弛试验,利用SPSS和Origin软件,开展蠕变本构方程中的材料常数拟合分析,并引入时效应力松弛预测模型,实现对应力松弛行为的预测。经不同应力水平下的预测结果与试验结果对比分析表明,该模型能较好地实现对时效成形过程中的应力松弛行为预测。     

基于参数Z_c预测25Cr35Ni耐热钢的蠕变行为与持久寿命

在不同温度T和不同应力σ下对25Cr35Ni耐热钢进行蠕变试验,引入参数P对不同蠕变应变下的温度和时间进行归一化处理,分析了(P,σ)数据点随蠕变应变的变化规律,引入参数Z_c分析了数据的分散程度并使用蠕变应变为5.0%下的试验数据对持久寿命进行了预测。结果表明:随蠕变应变的增加,σ-P曲线趋近于断裂曲线,Z_c先迅速增大后趋于稳定,(P,σ)数据点与其对应拟合曲线的偏差减小;基于Z_c参数法预测得到的持久寿命与实测持久寿命接近,数据分散程度较小。     

P92钢的蠕变-疲劳损伤行为及蠕变-疲劳损伤本构模型的建立

对P92钢在600℃下进行应力和应变控制的蠕变-疲劳试验,分析了载荷水平、保载时间对蠕变-疲劳损伤的影响;结合应力控制下的蠕变-疲劳试验数据,在黏塑性统一本构理论框架下引入修正的Chaboche非线性随动硬化率及蠕变应变并考虑损伤演化规律,构建了基于Chaboche理论的耦合蠕变-疲劳损伤本构模型,模拟了P92钢的蠕变-疲劳循环曲线.结果表明:P92钢在600℃下表现为循环软化特性;在应力控制下,P92钢高位保载的损伤与平均应力呈正相关,而低位保载的损伤与平均应力呈负相关;在应变控制下,P92钢产生应力松弛行为,保载时间越长,应力松驰越明显;建立的蠕变-疲劳损伤本构模型可以较好地模拟P92钢的循环特性,对于蠕变-疲劳过程中应力模拟的最大相对误差为7.30％.     

The sensitivity of the creep rupture of a pressurized cylinder to variations in the multiaxial rupture surface

The sensitivity of creep rupture life to variations in the multiaxial creep rupture surface is investigated for a long, thick-walled, internally pressurized circular cylinder. The cylinder is initially undamaged and accumulates damage at a rate dependent on both maximum principal stress and equivalent stress. It is shown that failure can initiate at the inner or outer radius, or at an intermediate radius, depending on the parameters in the creep and damage laws, and that the ratio of rupture time to initial failure time depends strongly on these parameters. The results are compared with estimates based on the mean diameter formula and in almost all cases a longer rupture life is predicted.     

Creep damage evaluation of thick-walled spheres using a long-term creep constitutive model

This paper describes a numerical model developed for the computation of creep damages in a thick-walled sphere subjected to an internal pressure and a thermal gradient. The model predicts the creep damage histories during the life of the sphere, owing to variations in stresses with time and through-thickness variations. The creep damage fraction is based on the Robinson’s linear life fraction damage rule, which has been incorporated in a nonlinear time-dependent stress analysis. Following the stress histories, the effective stress histories are obtained and the creep damages are calculated and summed during the life of the sphere. The material long-term creep properties up to the rupture and creep rupture data are defined by the Θ projection concept [1]. The damage histories up to 38 years are calculated and the results show that the maximum damages are always located at the inner surface of the sphere, while the outer surface of the vessel sustains minimum damages.     

高温结构应力松驰和蠕变损伤分析的实用方法

分析讨论了包含持续阶段的循环载荷作用下高温结构的应力－应变历史的规律性,在此基础上对现有的决定多轴应力松弛过程的简化方法进行了评述,并着重介绍了其简化方法。本文对该简化方法进行了改进,并用此改进形式对典型结构进行了应力松弛和蠕变损伤分析,与详细非弹性有限元分析结果的比较,证实所采用方法的可靠性和实用性。     

Omega蠕变寿命评估方法及应用

传统高温过热蒸汽管及过热炉管蠕变寿命评估多采用等温外推、Larson-Miller参数和θ参数等寿命评估方法,此类寿命评估方法常通过开展多组高应力蠕变测试进行短时外推,不仅耗时长、取样破坏性大、寿命评估存在分散性,而且此方法没有考虑材料的老化与损伤,长时外推结果置信度下降。1986年,美国材料学会(MPC)发起了用于评估工作在蠕变范围内部件剩余寿命的O-mega工程,目前该评估方法已被编入API 579 Fitness-For-Service标准中。通过介绍Omega寿命评估方法,并对1Cr0.5MoV和1.25Cr0.5MoSi两种材料开展Omega蠕变分析,再将Omega方法预测的材料蠕变断裂时间与实际断裂时间进行对比,发现Omega方法在评定蠕变寿命评估方面更具优越性。     

316L不锈钢扩散焊接头高温蠕变性能

以非标准持久试样进行了316L不锈钢扩散焊接头蠕变与持久试验。采用θ函数法描述和预测了316L不锈钢扩散焊接头蠕变曲线,并由θ函数法计算了最小蠕变应变速率和接头在6MPa,550℃条件下蠕变应变达到0.2%时的使用寿命,并将蠕变应变0.2%作为以316L不锈钢为母材的扩散焊构件高温结构设计标准。由持久试验结果可知,θ函数法与Larson-Miller法外推的316L不锈钢直接扩散焊接头蠕变断裂时间较为一致。     

Estimates of the creep rupture lives of structures subjected to cyclic loading

Constitutive relationships are discussed for materials which undergo creep rupture due to cyclic loading. Relationships are proposed which describe the strain rates in terms of the current stress and a single state variable. An approximate method is derived which enables a lower bound on the rupture life to be obtained for kinematically determinate structures subjected to cyclic load and isothermal conditions. The bound on the rupture life is expressed in terms of the energy dissipation rates within the structure corresponding to stationary-state creep. The effect of multi-axial stress creep-rupture upon the structural performance is examined and bounds are derived for materials which obey maximum tension and octahedral shear stress criteria. For both multi-axial stress rupture laws and structures subjected to cyclic and reverse load conditions formulae are derived which express the lower bound rupture life in terms of the behaviour of a steady-load uni-axial creep rupture test. Results of experiments which have been carried out on copper and aluminium beams are presented for cyclic and reverse load conditions. For both rupture laws the experimental rupture times are closely predicted by the representative rupture stresses and uni-axial data.     

Prediction of creep failure in aeroengine materials under multi-axial stress states

The creep and creep rupture behaviour of two, significantly different, aeroengine materials, namely a nickel-base superalloy at 700°C and a high temperature titanium alloy at 650°C, were studied. Experimental creep tests were conducted on uniaxial specimens and axisymmetric notched bars under constant tensile loads conditions. From the uniaxial creep test results, a creep continuum damage model was established for each of the materials. The skeletal point stress approach was used to obtain the approximate creep rupture stress criterion in the multi-axial generalization of the creep continuum damage models. This approximation was cross-checked using axisymmetric Finite Element (FE) analyses in a trial and error procedure. Multi-axial creep continuum damage models were then used in further FE creep analyses to predict the creep rupture times in specimens subjected to different tensile loads. The FE predictions of the rupture times in these notched specimens were found to be in good agreement with the experimental results for the nickel-base superalloy (Waspaloy) at 700°C and the titanium alloy (IMI834) at 650°C.     

A lower bound to the creep rupture time of pressurised thick cylinders

A lower bound to the creep rupture time of internally pressurised thick cylinders is derived. Material behaviour is described by a phenomenological creep rupture theory that accounts for all phases of creep, and for the full coupling between the deformation and damage processes. To obtain the desirable lower bound, the effective stress and the equivalent rupture stress, which represent the effects of multiaxial stress states on the creep strain and damage rates, respectively, were approximated by the maximum shear stress in the constitutive equations. By comparing the lower bound estimations for a wide range of cylinder dimensions and different engineering materials with the rupture times determined from accurate finite element calculations, it is shown that the lower bound estimations provide quite conservative lifetime predictions.     

保载应力和加载速率对316H不锈钢蠕变行为的影响

针对316H不锈钢,通过开展一系列高温蠕变试验,探讨了保载应力和保载前加载速率对材料蠕变行为的影响。结果表明,保载应力对材料蠕变行为影响较大,在进行高温结构蠕变损伤估算时,蠕变应变速率和蠕变断裂延性的选取均要考虑保载应力的影响;保载前的加载速率也对材料蠕变行为产生一定影响,尤其是加载速率较低时。因此,在进行高温结构蠕变损伤估算时,需考虑较低加载速率的影响。